The invention relates to mechanisms for transmitting mechanical forces and particularly to mechanisms for transmitting motion which allow a manual override. The prior art includes various mechanisms that will transfer the drive means for a given apparatus from a manual apparatus to a motor driven apparatus. An example of an apparatus which is generally of this type is disclosed in U.S. Pat. No. 3,791,071. The apparatus includes an axially movable member which is moved to vary the drive for a window in an automobile between a manual drive and an electric motor drive. A disadvantage of such apparatus is that there is no position reference to permit accurate control of an apparatus in a wide number of discrete positions. In the apparatus in the above-referred to patent, a D.C. motor is ordinarily used to power a window mechanism until a physical limit switch interrupts power to that motor. For an increasing number of applications it is highly desirable to use a digital computer to drive a stepping motor which is driven by pulsed direct current power. The stepping motor frequently will be driven through a plurality of discrete steps with each of the steps being of equal angular extent which can be determined in advance. This permits the operation of the motor in an open loop mode without compromising the precision with which the apparatus is positioned.
In the automotive field the more stringent requirements for emissions control and fuel economy have made it highly desirable to use a digital computer to control various mechanisms. One such mechanism is the choke plate in the carburetor of a typical internal combustion engine. If an ordinary mechanical drive is used to couple a stepping motor and the choke plate, there are various conditions when the mechanism is vunerable to damage. One such condition occurs when there is a backfire through the carburetor. It is conventional to mount the choke plate in a carburetor with the axis of the choke plate asymmetric with respect to the geometric axis of the choke plate. The passage of a large amount of gas and vapor as a result of a backfire will inherently produce a rotational moment about the axis of the mounting shaft for the choke plate. This moment will tend to damage any gear train that is mechanically connected to the mounting shaft of the choke plate and an associated stepping motor. In some cases it may also move the stepping motor a number of discrete steps and this will result in the digital computer acting on the basis of an assumed initial position which will be incorrect. This follows since the computer is operating in an open loop mode. Other circumstances where such positional changes and damage to the gear train may occur are when a mechanic forceably positions the choke plate toward either an open or a closed position to either start an automobile or to conduct some other tests.
Another reason for using digital computers to drive stepping motors in automobiles is that the large number of pneumatic controls now required to meet emissions and economy standards has created a problem in that the vacuum available may not be sufficient to reliably and consistently operate all of the various controls under all of the various operating conditions of the engine. A related problem is that a large number of tubes connected to the intake manifold are also vunerable to leakage and accordingly disrupt the normal operations of the various mechanisms.
It is an object of the invention to provide apparatus which will provide the advantage of the precise mechanical positioning of apparatus such as a choke plate of a carburetor with provision for mechanical override of that mechanism for a finite period of time after which the apparatus will return to either the original position or any new position which is dictated by an input from a stepping motor. Stated another way the override mechanism will have a memory.
It is another object of the invention to provide apparatus which allow manual override in both angular directions.
It is yet another object of the invention to provide apparatus which will precisely control the position of a choke plate or other similar apparatus in a manner which will not be subject to variation because of natural forces such as the flow of air into the carburetor which will produce a moment about the mounting shaft for the choke plate because of the asymmetric location of the mounting shaft for the choke plate.